In Japan, techniques for increasing or decreasing the amount of gas dissolved in liquid are divided into gas dissolving techniques and deaeration techniques. Thus, the prior art methods are described separately.
Techniques for dissolving gas include: a method in which an ejector nozzle is attached in a closed tank, and then gas and liquid are mixed under a fixed pressure so as to contact with each other and whereby become dissolved; and a gas dissolving device disclosed in Japanese Published Unexamined Patent Application No. 2003-190750 (Patent Document 1, hereinafter) in which liquid injected from a nozzle collides against a whirl stop arranged in a tank so that the gas is dissolved owing to the pressure of the collision.
Further, in a gas-liquid mixture dissolving device of Japanese Published Unexamined Patent Application No. 2000-161278 (Patent Document 2, hereinafter), disclosed are: a method in which liquid is injected in the form of waterdrops through a nozzle from the upper part of a pressurized tank so that gas is dissolved into the waterdrops; a method in which gas and liquid are mixed in a pressurized tank so as to be dissolved; a method in which a plurality of centrifugal pumps are connected in series so that the pump discharge pressure is increased, so that the gas suction rate is increased, and so that gas and liquid are mixed and dissolved in the centrifugal pumps. Further, a method has been developed in which fine air bubbles are generated directly in liquid so that the contact area of the gas and the liquid is increased, whereby dissolution is achieved.
On the other hand, techniques for deaerating gas include: a vacuum deaeration method in which liquid is sprayed and falls from the upper part of a filler packed in a deaeration column, so that dissolved gas is removed while maintaining the degree of vacuum; and a deaerator disclosed in Japanese Published Unexamined Patent Application No. 2000-325703 (Patent Document 3, hereinafter) in which throttle valves are provided in a pipe on the suction side of a pump and in a tank, and then when the pump operates, the pressure is reduced in the pipe and the tank so that gas dissolved in the liquid is caused to generate bubbles, and then the fluid to be treated is circulated depending on a desired gas recovery rate so that deaeration is achieved.
Nevertheless, the above-mentioned conventional techniques have the following problems.
In the method in which liquid and gas are mixed in a pressurized tank thereby the contact frequency of the liquid and the gas is increased so that dissolution is achieved, energy (pressure and mixture) for dissolution is repeatedly applied even to a liquid portion having already reached a dissolution saturation value under the present pressure. This causes the problem of a remarkable inefficiency that a desired dissolution concentration beyond the saturation value of the dissolution concentration is to be obtained. Further, the method disclosed in Patent Document 1 in which liquid collides against a whirl stop so that gas is dissolved has the problem that the pressure itself generated at the time of collision is not effectively used as energy for dissolving the gas into the liquid, and hence that the gas cannot be dissolved to a high concentration.
Further, in the method in which waterdrops are injected from the upper part of a pressurized tank, the gas is dissolved in proportion to the gas pressure solely in the surface part of the waterdrop, while the gas is not dissolved into the center of the waterdrop. This causes the problem that the dissolution concentration of the treated water cannot be increased efficiently.
The method in which gas and liquid are mixed in a pressurized tank so as to be dissolved has the problem that a large amount of gas is used while continuous operation cannot be performed unless the non-dissolved gas is discharged to the outside of the container, and that the dissolution concentration is low while the economical efficiency is also low.
In the method disclosed in Patent Document 2 in which a plurality of centrifugal pumps are connected in series whereby a high pressure is generated so that the amount of mixed gas is increased so that dissolution is achieved, when the pump discharge pressure is increased, the gas in the pump is compressed and hence the volume becomes small. Thus, a larger amount of gas can obviously be sucked from the suction side of the pump. Nevertheless, mixing a larger amount of gas does not necessarily result in efficient dissolution of the gas. Thus, this causes the problem of a low processing rate and low concentration in spite of the use of a high energy.
Further, in the method in which fine air bubbles are generated directly in liquid, gas dissolution is achieved solely by the effect of an increase in the contact area of the liquid and the gas. Thus, the gas dissolution concentration becomes exceedingly difficult to make it greater than the saturated concentration in the site. Thus, this causes the problem that high concentration gas dissolving liquid is difficult to be obtained.
On the other hand, in the vacuum deaeration method, fluid to be treated is sprayed and falls from the upper part of a tank in a reduced pressure state, so that deaeration is achieved. This method is effective in the case that pure water or the like is to be generated from water having a small amount of mixing of solid foreign substances. Nevertheless, for example, in the case of liquid such as natural environment water and factory waste water having a large amount of mixing of solid foreign substances, the problem arises that clogging frequently occurs in the discharge nozzle and the filler, and hence that the method is not appropriate for continuous operation. Another problem is that the processing rate is low, and hence that a large amount of fluid to be treated cannot be processed. Further, in the invention disclosed in Patent Document 3, even when the fluid to be treated is brought into a reduced pressure state by the suction force in the suction side of the pump, the contact area of the liquid and the gas space is not securely increased, so that the gas dissolved in the fluid to be treated does not efficiently achieve reduced pressure foaming. Thus, unless the fluid to be treated is circulated so that the gas is removed repeatedly in a reduced pressure state, desired deaeration treated fluid cannot be obtained. This causes a problem of a high cost.
The invention has been made in view of these situations in the prior art. An object of the invention is to provide a method, a device, and a system for controlling a dissolved amount of gas in which fluid to be treated introduced into a reduced pressure space or a pressurized space so that successively-generated-and-broken liquid bubbles (an aggregate of air bubbles in the form of soap bubbles) of the fluid to be treated are generated in the space, so that the contact area of the gas and the liquid is increased to a large extent, whereby in the case of a reduced pressure, the gas dissolved in the liquid can be released efficiently, while in the case of pressurization, the gas can be dissolved efficiently into the liquid without a loss so that high concentration gas dissolving liquid can be generated that has a concentration near the pressure-corresponding dissolution concentration given by Henry's law, and in which the effect of gravity is utilized to a maximum extent so that an outstanding energy saving property is realized in the control of the dissolved amount of gas in the liquid.